This invention relates to papermaking processes. More specifically it relates to a binder system comprising a highly dispersible, high specific surface area calcium carbonate (colloidal PCC) and a cationic starch and the use of said system to produce paper having improved levels of retention of fillers, and improved strength and opacity.
In its efforts to produce high quality paper at the lowest possible price and to minimize environmental problems associated with disposal of the large volumes of waste chemicals resulting from paper manufacture, the paper industry has resorted to a variety of approaches. These include the use of lower quality pulps and/or the use of mineral fillers as substitutes for cellulose fibers. However, such approaches tend to reduce the strength, filler retention, dry brightness and other properties of the resulting paper, often with no beneficial effect upon environmental problems; and, of course, no economic advantages.
Attention is directed to Volume 16 of the KirkOthmer "Encyclopedia of Chemical Technology", pages 768-825 (1981), John Wiley & Sons, N.Y., for a summary of paper making technology, including reference to precipitated calcium carbonate as a filler in paper making.
Fillers are added to the pulp slurry during the paper making process to improve various properties such as softness, smoothness, brightness and opacity. Typical fillers are titanium dioxide, calcium carbonate, talc, synthetic silicates and clays such as kaolin and bentonite. As regards fillers in general, and calcium carbonate in particular, their retention; i.e., filler retention, is a function of their particle size, specific surface area, surface charge and specific gravity. Calcium carbonate, more specifically precipitated calcium carbonate (PCC), is enjoying increasing use in the paper industry as a filler as a result of the development of on-site PCC satellite plants; i.e., plants which produce PCC at or close by the paper mill in which it is to be used, making use of the carbon dioxide produced by the paper mill to convert calcium hydroxide to PCC.
In order to improve operation of the paper making machine and to improve the quality of the paper, various chemicals, generally referred to as processing aids are added to the paper. These include retention aids, flocculants, drainage aids, defoamers and slimicides. Retention aids are used to improve filler retention in the papermaking process by coflocculating the filler with pulp fibers and fines. Typical retention aids are amino or quaternary ammonium group containing polymers such as condensation polymers of diethylenetriamine-adipic acid polyamide which are treated with epichlorohydrin and ammonia- or dimethylamine-epichlorohydrin condensates and polyacrylamides (PAM). However, PAM retention aids are shear sensitive, can over flocculate a sheet causing poor formation and reduced drainage on the paper machine wire resulting in reduced productivity.
Starches are used in the papermaking process to improve dry strength of the paper. Also used are modified starches especially those having tertiary amino or quaternary ammonium groups and which are cationic in nature.
Recent developments in the paper industry which tend to overcome the above-mentioned deficiencies involve the use of binder systems such as combinations of anionic retention agents, e.g. polyacrylamides, and cationic starch. The most recent development as regards binder systems comprises a combination of cationic starch and anionic colloidal silicic acid. Such binder systems are described in U.S. Pat. Nos. 4,385,961 and 4,388,150, issued May 31, 1983 and June 14, 1983, respectively. While these binder systems, particularly those comprising cationic starch and anionic colloidal silicic acid, result in paper having improved filler retention and dry strength, relative to paper made in the absence of said systems, they do not enhance the opacity of the finished paper because of over flocculation of the filler and generally increase the cost of papermaking.